1. Field of the Invention
This invention relates, in general, to navigation apparatus, and specifically, to apparatus for determining the coordinates of a predetermined position on a navigation chart.
2. Description of the Prior Art
Various methods and devices have been developed for determining the position of a ship or object on the surface of the earth. In determining the coordinates of the position on the surface of the earth, various sightings and other mesurements are taken to determine the exact longitude and latitude of the ship or object. These coordinates are applied to a navigation chart which is divided into spaced latitude grid lines and spaced longitude grid lines. However, due to the large spacing on some of the charts between the adjacent longitude and latitude lines an interpolation scale having incremental markings dividing the distance between two adjacent grid lines is used to locate the exact position of the object between the two spaced latitude and longitude lines.
Another navigation system in widespread use is the so-called long range navigation system or Loran in which pairs of transmitters are positioned on the surface of the earth and transmit timed pulses which are received by Loran receivers mounted on ships or other moving objects. The difference in arrival time of the pulses from the pair of transmitters is then used to locate the position of the ship or object along two spherical hyperbolas having their loci at the respective pairs of transmitters. The intersection of the two hyperbolas fixes the position of the object.
Charts are provided having families of hyperbolas plotted thereon corresponding to each pair of transmitters. The hyperbolas for each transmitter are spaced apart on the chart at a predetermined time difference or interval, typically one hundred microseconds. The navigator needs only take the indicated time difference of the signal received from one of the pair of transmitters and interpolate the difference between two adjacent hyperbolas of the family of hyperbolas corresponding to the transmitter closest to the measured time difference for that family of hyperbolas.
An interpolation scale printed on the chart provides scaled increments between adjacent time delay interval lines on the chart. Using a pair of dividers or a ruler, the navigator need only measure the indicated time difference on the interpolation scale and plot the measured distance on the chart to determine one position coordinate.
Although automatic navigation apparatus have been devised which automatically determine the exact position coordinates, either latitude or longitude or Loran time intervals, such equipment is expensive and it is still necessary to plot the position on the chart by using interpolation scales. In using interpolation scales, repeated measurements by means of a divider or ruler are required to interpolate the position between the adjacent grid lines. This is, at best, a time consuming task and one which is prone to numerous measurement errors.
Thus, it would be desirable to provide a navigation apparatus for plotting the position of an object which overcomes the problems of previously devised navigation position plotting apparatus. It would also be desirable to provide a navigation apparatus which determines the interpolated latitude and longitude or Loran time interval coordinates of a predetermined position on a chart between adjacent pairs of latitude and longitude lines or Loran time interval lines. It would also be desirable to provide a navigation apparatus which enables the position of an object to be plotted once the latitude and longitude or Loran time interval coordinates are known. Finally, it would be desirable to provide a navigation position plotting apparatus in which the position coordinates of an object or point on a navigation chart are easily obtained with a minimum number of steps.